Physicochemical Characterization of Nanofillers for Unsaturated Polyester Resin Modification

Nanofillers are widely used to enhance the performance of polymer composites; however, their effectiveness strongly depends on their physicochemical properties and dispersion behavior. In this study, five commercially available nanofillers, namely multi-walled carbon nanotubes (CNTs), nanoclay, halloysite, Cloisite® 30B, and zinc oxide (ZnO), were comprehensively characterized as potential modifiers of unsaturated polyester resin. The results confirm the characteristic chemical composition and crystalline structure of all investigated nanomaterials. CNTs exhibited the highest specific surface area (213.7 m2 g−1) and pore volume, whereas ZnO showed the lowest median equivalent particle diameter determined by laser diffraction (D50 = 2.09 μm). The clay-based fillers displayed comparable particle size distributions, with D50 values ranging from 8.29 to 9.00 μm. SEM observations revealed substantial differences in morphology and agglomeration state, with CNTs forming compact entangled agglomerates, while ZnO exhibited the most homogeneous microstructure. Suspension stability analysis demonstrated that particle size alone could not explain the observed sedimentation behavior. ZnO exhibited the highest dispersion stability, whereas halloysite and Cloisite showed progressive sedimentation. Nanoclay displayed delayed destabilization associated with agglomeration processes, while CNT suspensions were governed by structural rearrangements rather than classical sedimentation. The obtained results indicate that the dispersion behavior of nanofillers in unsaturated polyester resin is associated with differences in particle size, morphology, and agglomeration tendency. The presented comparative characterization provides a basis for the rational selection of nanofillers for polyester resin-based nanocomposites.